Quoting Magic Banana: I had said: Between the three HDD's, my T420 now has five Trisquel OS's ...
Followed by MB's optimistic reply: "Don't you want just one?! You can upgrade one Trisquel 7 that is on the internal hard drive (faster) with 'do-release-upgrade -d' (no need for a fresh install) and get rid of the remaining systems (not the user data!)."
Maybe too soon ? Trisquel_8 is still young and in need of experience with its many applications. Way back when Ubuntu's recent upgrade debuted, it came to me as a simple choice to be installed right alongside the usual apt-get update/upgrade, and that transition went flawlessly ... but I just keep up with the Ubuntu installation "just in case" on a backup desktop for which Trisquel is the main operating system, so I cannot say whether that transition was trouble-free or troublesome.
Now spring forward to Spring 2018, and Trisquel's cleansing of Ubuntu is ready for trial among the masses (e.g.,me). I'm not ready to make that leap of faith in _my_ ability to get Trisquel_8 working on the T420's heavily burdened internal HDD without trashing all the applications & data that are working just fine. HDD's are quite inexpensive when compared with all the other tasks in my life. I am actually quite pleased with the ease with which I got it all put together in about a day.
My experience with Debian and earlier versions of linux was like that in a Dickens novel by comparision ... careful prior preparation of various special-purpose partitions according to detailed instructions, followed by an out-of-the-blue actual installation bearing no resemblance to the prior instructions ... too many pitfalls that I encountered, many if not most of my own making ... now there is the fully develplace this particular version of linux (Trisquel) at the pinnacle of operating systems that helps me use my T420 laptop with close to zero worries or hassles with software while dealing with the hassles presented by life itself.
The only hassle with the Trisquel installation that I faced was the less-than-obvious requirement to set the target location as root (/), requiring that I guess that location of the partition menu needed to make that adjustment is hidden behind a plus (+) sign in the otherwise uncluttered menu. The only hassle with etcher.io was the utter lack of any installation instructions on its uncluttered homepage ... resolved by the helpful raspberry.pi folks as already described.
I use my T420 a lot ... as evidenced by the absence of several letters on its qwerty keyboard and by the CD tray that pops out just about every time I hit the Enter key.